In recent years, with the increasing concerning in a global warming in various quarters in Japan and abroad, various efforts have continued for discharge control of carbon dioxide. An increase in the consumption of fossil fuels brings an increase of carbon dioxide released into the atmosphere. Due to the greenhouse effect, the atmospheric temperature of the earth rises, and it makes a serious influence to the environments of the earth. In order to solve the global problems, various approaches have been examined. In particular, anticipation for photovoltaic generation is increasing, because it is clean and does not generate environmental pollution.
The solar cell constitutes a core part of the solar energy power generation system which directly changes the energy of sunlight into the electrical energy, and which is made of a semiconductor of a single crystal, polycrystal, amorphous silicon, etc. A solar cell element (cell) is not used alone. In general, several to several dozen of the solar battery elements are wired in series or in parallel, and the solar battery elements are packed to be unitized in order to protect the cell over a long period of time (about 20 years). The unit built in the package is called a solar cell module.
As shown in FIG. 1, the solar cell module includes a surface exposed to the sunlight which is covered with a glass A-3, gaps separating the solar cell elements A-1 are filled with a thermoplastic A-2, in particular, a filler including an ethylene-vinyl acetate copolymer, and the rear surface thereof is protected by a sealing sheet B.
Since the solar cell modules are used out doors, the structure, material to be used, etc. are required to have sufficient durability and weather resistance. In particular, the sheet for sealing a rear surface of a solar cell is required to have weather resistance as well as a small moisture vapor transmission rate (that is, excellent moisture barrier properties). When a filler is peeled, or discolored, or wiring is corroded, by moisture transmission, there is possibility that electrical output power itself of the module is influenced.
In the past, a fluorine resin which has excellent adhesive to an ethylene-vinyl acetate copolymer which has excellent weather resistance and fire resistance and has been used as a filler for the solar cell module (Patent Document Nos. 1 and 2). In addition, a sheet for sealing a rear surface of a solar cell which includes a film made of polyester having excellent electric insulation, such as polyethylene terephtharate, has also been developed. Furthermore, in order to improve inferior weather resistance, which is a disadvantage of the polyester film, a composition comprising an ultraviolet absorber was suggested (Patent Document No. 3), attempts in which an amount of a circular oligomer in polyester is adjusted (Patent Document Nos. 4 and 5), and in which the molecular weight of polyester is adjusted (Patent Document No. 6), were suggested.
As explained above, the solar cell is required to maintain its performance for about twenty years. In order to evaluate the durability, an acceleration test was performed under a high temperature and a high moisture (85° C.-85% relative humidity) environment. In particular, when a polyester film is used as a substrate, the film substrate is hydrolyzed in the acceleration test, and the strength of the film is dramatically decreased. Therefore, as explained in Patent Documents, it is very much a situation in which various attempts were made to compensate the decrease of the strength. However, there is a new problem that is caused by improving the weather resistance (hydrolysis resistance) of the film substrate.
The problem is delamination which is caused by hydrolysis of an adhesive for attaching the film substrate. In general, when heat resistance and safety are concerned, a polyurethane-based adhesive comprising polyester polyol as a main component is used. However, polyester polyol is hydrolyzed and the molecular weight thereof becomes smaller. Thereby, a problem, in which adhesive strength is remarkably decreased under the conditions of the acceleration test, is highlighted.
The problem is similarly happened in the fluorine resin which is explained above. When the acceleration test was conducted, not only is the surface raised by delamination with time and the appearance inferior, but the barrier properties which are required for the sheet for sealing a rear surface are also decreased. These may affect the electric output of the solar cell.
It is very much a situation in which not only is the film substrate required to have weather resistance (hydrolysis resistance) but also the adhesive for attaching the film substrates is required to have hydrolysis resistance. However, these requirements have not been achieved yet.
[Patent Document No. 1]: Japanese Language publication (Kohyo) 8-500214 corresponding to PCT Application
[Patent Document No. 2]: Japanese Language publication 2002-520820 corresponding to PCT Application
[Patent Document No. 3]: Japanese Patent Application, First Publication No. 2001-111073
[Patent Document No. 4]: Japanese Patent Application, First Publication No. 2002-100788
[Patent Document No. 5]: Japanese Patent Application, First Publication No. 2002-134771
[Patent Document No. 6]: Japanese Patent Application, First Publication No. 2002-26354